Temperature sensitive (ts) mutants have been isolated in Balb/c 3T3 cells in culture. The ability of mutants in DNA synthesis to support oncogenic papovavirus infection will be studied: replication by polyoma, transformation by Simian virus 40. Additional ts mutants are being sought. Synchronized permissive cells will be evaluated for virus replication, transformation, and induction of the viral genome from transformed cells. Enzymatic studies with DNA endonucleases and DNA polymerases will be performed.